Porous membrane and method for manufacturing the same

ABSTRACT

A method for manufacturing a porous membrane including a three-dimensional network structure and a spherical structure is provided. The method includes forming a porous membrane having a spherical structure, applying a resin solution onto at least one surface of the porous membrane having the spherical structure, followed by immersing the membrane in a solidification liquid, thereby forming the three-dimensional network structure on at least one surface of a porous membrane having the spherical structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 10/448,122, filed May 30, 2003, now U.S. Pat. No. 7,258,914 B2,the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to micro filtration membranes andultrafiltration membranes used for water treatment, such as drinkingwater production, water purification, and effluent treatment. Thepresent invention also relates to porous membrane modules and to waterseparation apparatuses including such a porous membrane. Furthermore,the present invention relates to battery separators, charged membranes,fuel cells and blood purification membranes using a porous membrane.

2. Description of the Related Art

Porous membranes have been used in various fields including watertreatment, such as water purification and effluent treatment; medicalapplication, such as blood purification; food engineering; batteryseparators; charged membranes; and fuel cells. In the field of producingdrinking water, that is, in use for water treatment, such as waterpurification and effluent treatment, separation membranes are beingsubstituted for conventional sand filtration and coagulationsedimentation, and are being used to improve the quality of treatedwater. A large amount of water is treated in these fields. Accordingly,a separation membrane having an excellent water permeability isadvantageously used in view of membrane replacement costs and thefootprints of apparatuses because an excellent water permeability canlead to a reduced area of the membrane, consequently reducing the sizeof the apparatuses and saving equipment expenses. The separationmembranes are also required to have chemical resistance. In waterpurification, in order to sterilize permeate and prevent biofouling ofmembranes, an antiseptic, such as sodium hypochlorite, is added or themembranes are washed with an acid, an alkali, chlorine, a surfactant, orthe like. Accordingly, separation membranes using a chemical-resistantmaterial, such as a polyethylene resin, a polypropylene resin, or apolyvinylidene fluoride resin, have recently been developed and put intouse. In the water purification field, accidents have surfaced since1990s in which chlorine-resistant pathogens, such as cryptosporidiumderived from livestock excrements or the like, are not completelydisposed of in a filtration plant and, thus, contained in treated water.In order to prevent such an accident, separation membranes are requiredto have high physical strength and sufficient separation properties toprevent raw water from contaminating treated water.

In medical application, porous membranes are being used for bloodpurification, hemodialysis particularly serving as a substitution forkidney functions, blood filtration and blood filtration dialysis, andremoval of waste products in blood by extracorporeal circulation. Infood industry, porous membranes are used, in some cases, to separate andremove yeast used for fermentation and liquid condensation. In the fieldof batteries, porous membranes are being used for battery separatorsthat allow electrolytes, but not cell reaction products, to permeatetherethrough. Also, in the field of fuel cells, some porous membranesare used as the base material of macromolecular solid electrolytes. Onthe other hand, in the ultrapure water production, charged porousmembranes are used to enhance ion exclusion characteristics and thepurity of produced water, in some cases.

These porous membranes are required to have excellent separationcharacteristics, high chemical and physical strengths, and an excellentpermeability, which shows how much untreated liquid permeates throughthe membranes.

European Patent Application No. 0037836 has disclosed a wet solutionmethod for forming an asymmetrical porous structure by nonsolventinduced phase separation. In this method, a polymer solution prepared bydissolving a polyvinylidene fluoride resin in a good solvent is extrudedfrom an extrusion head at a temperature much lower than the meltingpoint of the polyvinylidene fluoride resin or is cast on a glass platefor forming. The product is brought into contact with a liquidcontaining a nonsolvent for polyvinylidene fluoride resins. In the wetsolution method, unfortunately, it is difficult to perform uniform phaseseparation in the thickness direction and the resulting asymmetricalmembrane has macro voids. Therefore, the mechanical strength of themembrane is unsatisfactory. Also, many factors of membrane formingconditions influence the structure and characteristics of resultingmembranes. It is, therefore, difficult to control the step of formingmembranes and reproducibility is poor. U.S. Pat. No. 5,022,990 hasrelatively recently disclosed a melt-extraction method for forming aporous structure. In this method, a polyvinylidene fluoride resin ismelt-kneaded with inorganic particles and an organic liquid. The mixtureis extruded from an extrusion head at a temperature higher than or equalto the melting point of the polyvinylidene fluoride resin or is pressedwith a pressing machine, for forming. After cooling, the organic liquidand the inorganic particles are extracted. Thus, the porous structure isformed. This melt-extraction facilitates the control of voidcharacteristics and helps prepare relatively uniform, strong membraneswithout forming macro voids. However, if the inorganic particles are notdispersed well, a defect, such as a pin hole, can occur. Also, themelt-extraction undesirably increases manufacturing costs extremely.

Other techniques for manufacturing a porous membrane have been disclosedin which polyolefin resins, such as polyethylene and polypropylene, areused as raw materials. For example, a polyolefin film containing aninorganic filler is drawn in at least one direction so that interfaceseparation occurs between the inorganic filler and the polyolefin toform voids in the film (for example, Japanese Unexamined PatentApplication Publication Nos. 7-26076 and 9-25372). In this technique,however, since the inorganic filler must be extracted to be removed,manufacturing costs increase undesirably. Furthermore, in thistechnique, it is difficult to control the pore size in membrane surfacesand, therefore, only membranes having a relatively large pore size of0.1 to 1.0 μm are manufactured.

European Patent Application No. 0245863 illustrates a composite membraneincluding an ultrafiltration membrane disposed on a porous membrane. Inthe preparation of this composite membrane, the porous membrane, actingas the base material, is treated with an alcohol solution of glycerin toenhance the affinity for the ultrafiltration membrane. After drying, apolymer solution is applied to the base material and is solidified toform the ultrafiltration membrane. This technique therefore makesmanufacturing processes complicated and extremely increasesmanufacturing costs.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a porous membranehaving a high strength and excellent water permeability and rejectionproperties.

The present invention is directed to a porous membrane having both athree-dimensional network structure and a spherical structure.

The present invention is also directed to a water separation apparatusincluding a porous membrane module using the above-described porousmembrane and compression means at the raw water side of the porousmembrane module or suction means at the permeate side.

The present invention is also directed to a method for producingpermeate from raw water using the water separation apparatus.

The present invention is also directed to a battery separator, a chargedmembrane, a fuel cell, and a blood purification membrane using theporous membrane.

The present invention is also directed to a method for manufacturing aporous membrane having both a three-dimensional network structure and aspherical structure. In the method, a thermoplastic resin is dissolvedin a solvent. The resulting resin solution is discharged from anextrusion head into a cooling liquid to be solidified. In this instance,different compositions of the cooling liquid are respectively uses forone surface side of the porous membrane and the other surface side.

The present invention is also directed to a method for manufacturing aporous membrane having both a three-dimensional network structure and aspherical structure. In this method, the three-dimensional networkstructure is formed on at least one surface of a porous membrane havingthe spherical structure.

The present invention is also directed to a method for manufacturing aporous membrane having both a three-dimensional network structure and aspherical structure. In this method, a resin solution for forming thethree-dimensional network structure and a resin solution for forming thespherical structure are simultaneously discharging from an extrusionhead, and are subsequently solidified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional photograph of a hollow fiber membranemanufactured by a method in Example 1 according to the presentinvention.

FIG. 2 is a sectional photograph of the area around the external surfaceof the hollow fiber membrane manufactured by the method in Example 1according to the present invention.

FIG. 3 is a sectional photograph of the area around the interior surfaceof the hollow fiber membrane manufactured by the method in Example 1according to the present invention.

FIG. 4 is a cross-sectional photograph of a hollow fiber membranemanufactured by a method in Comparative Example 2 in association withthe present invention.

FIG. 5 is a sectional photograph of the area around the external surfaceof the hollow fiber membrane manufactured by the method in ComparativeExample 2 in association with the present invention.

FIG. 6 is a sectional photograph of the area around the interior surfaceof a hollow fiber membrane manufactured by the method in ComparativeExample 2 in association with the present invention.

FIG. 7 is a sectional photograph of the area around the external surfaceof a hollow fiber membrane manufactured by a method in ComparativeExample 23 in association with the present invention.

FIG. 8 is a sectional photograph of the area around the external surfaceof a hollow fiber membrane manufactured by a method in Example 26according to the present invention.

FIG. 9 shows an example of a hollow fiber membrane module.

FIG. 10 shows an example of a water separation apparatus using a hollowfiber membrane module.

FIG. 11 shows an example of a spiral element.

FIG. 12 shows an example of a plate-and-frame element.

FIG. 13 is a schematic illustration of an MEA of a direct methanol fuelcell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A porous membrane of the present invention is characterized in that ithas both a three-dimensional network structure and a sphericalstructure. The three-dimensional network structure here refers to astructure in which solid contents spread in three dimensions. Thethree-dimensional network structure has pores separated by solidcontents forming a net.

The spherical structure here refers to a structure in which manyspherical or substantially spherical solid contents are combined to eachother directly or through streak solid contents. Supposedly, thespherical structure substantially consists of spherulites. A spheruliteis thermoplastic resin crystals precipitated and solidified when athermoplastic resin solution is phase-separated to form a porousstructure.

The mean pore size of the three-dimensional network structure ispreferably in the range of 5 nm to 50 μm, and more preferably in therange of 10 nm to 30 μm. The mean pore size of the three-dimensionalnetwork structure refers to the mean diameter of the pores in thethree-dimensional network structure. In order to determine the mean poresize, the cross section of the porous membrane is photographed through ascanning electron microscope (SEM) or the like at a magnificationallowing the pores to be clearly observed, and the diameters ofarbitrary 10 or more pores, preferably arbitrary 20 or more pores, aremeasured and number-averaged. Also, the mean pore size may be determinedusing an image processing system in which the mean diameter of the poresis measured. In this instance, the mean diameter of equivalent rounds isdefined as the mean pore size. The mean diameter of equivalent rounds isdetermined by the expression (a×b)^(0.5), wherein a and b are thebreadth and the length of elliptical pores, respectively.

The mean diameter of the spherical structure is preferably in the rangeof 0.1 μm to 10 μm, and more preferably in the range of 0.2 μm to 5 μm.In order to determine the diameter of the spherical structure, thesurface or cross section of the porous membrane is photographed througha scanning electron microscope or the like at a magnification allowingthe spherulites to be clearly observed. The diameters of arbitrary 10 ormore spherical structures, preferably arbitrary 20 or more sphericalstructures, are measured and number-averaged. The mean diameter may bedefined as the mean diameter of equivalent rounds obtained by analyzingthe photograph with image processing system.

A membrane having the spherical structure results in a strong membranewith the permeability maintained. However, it is not easy to enhance therejection properties. By giving the porous membrane of the presentinvention both a three-dimensional network structure and a sphericalstructure together, the resulting porous membrane exhibited a highstrength, a high permeability, and high rejection properties. Inparticular, by setting the mean pore size of the three-dimensionalnetwork structure in the range of 5 nm to 50 μm and the mean diameter ofthe spherical structure in the range of 0.1 to 10 μm, the strength, thepermeability, and the rejection properties are advantageously broughtinto balance at a high level. A three-dimensional network structureincluding macro voids particularly with a mean pore size of more than 50μm can lead to a membrane having an excellent permeability. However, thestrength of the resulting membrane becomes low.

The three-dimensional network structure and the spherical structure maycoexist in any form. Preferably, in order to bring into balance thestrength, the permeability, and the rejection properties at a highlevel, the three-dimensional network structure and the sphericalstructure are layered on each other. Particularly preferably, thethree-dimensional network structure is disposed on one surface side ofthe membrane and the spherical structure is disposed on the other side.

Preferably, the porous membrane of the present invention has a waterpermeability in the range of 0.1 to 10 m³/m²·h at 50 kPa and 25° C. anda rejection of 90% or more for particles with a particle size of 0.843μm. Preferably, it also exhibits a fracture strength of 2 MPa or moreand a fracture elongation of 15% or more. The water permeability is morepreferably in the range of 0.15 to 7 m³/m²·h. The rejection is morepreferably at least 95% for particles with a particle size of 0.843 μm.The fracture strength is, more preferably, at least 3 MPa. The fractureelongation is, more preferably, at least 20%. If these requirements aresatisfied, a porous membrane can be achieved which has sufficientstrength, permeability, and rejection property to be used in watertreatment, battery separators, charged membranes, fuel cells, bloodpurification, and the like.

The porous membrane of the present invention may suitably be used in ahollow fiber form or a flat form.

The measurements of water permeability and rejection properties wereperformed on a miniature module of 200 mm in length including fourhollow fiber membranes. Reverse osmosis membrane treated water wasentirely filtered for 30 minutes by external pressure at a temperatureof 25° C. and a differential pressure of 16 kPa. Thus, a quantity ofpermeate (m³) is measured. The quantity of permeate (m³) is convertedinto a value per hour (h) and a value per effective membrane area (m²).These values were further multiplied by 50/16 and converted into a valueat a pressure of 50 kPa. Thus, the water permeability was determined.Water in which polystyrene latex particles having a mean particle sizeof 0.843 μm were dispersed was entirely filtered for 30 minutes byexternal pressure at a temperature of 25° C. and a differential pressureof 16 kPa. The rejection properties can be determined from the ratio ofthe latex particle concentration in raw water to that in permeate. Theselatex particle concentrations are obtained by measuring absorptioncoefficients of ultraviolet light having a wavelength of 240 nm. Themeasurements on a flat membrane are performed in the same manner as inthe measurement on the hollow fiber membrane, except that the membraneis cut to a circle of 50 mm in diameter and the circle membrane isplaced on a cylindrical filtration holder. The water permeability may bederived from a value obtained under pressure or aspiration. Watertemperature may be estimated from the viscosity of liquid to beevaluated. A water permeability as low as less than 0.1 m³/m²·h is notsuitable for the porous membrane because such a water permeability isexcessively low. In contrast, if the water permeability is as high asmore than 10 m³/m²·h, the porous membrane has such an excessively largepore size that the impurity rejection properties are negativelyaffected. Also, when the rejection is less than 90% for particles with aparticle size of 0.843 μm, the porous membrane undesirably has anexcessively large pore size and degraded impurity rejection properties.

The fracture strength and the fracture elongation can be measuredwithout particular limitation. For example, a tensile test is performedwith a tensile tester at a tensile speed of 50 mm/min on more than fivesamples having a measurement length of 50 mm. Obtained fracturestrengths and fracture elongations are averaged. If the fracturestrength is less than 2 MPa or the fracture elongation is less than 15%,the porous membrane is difficult to handle and is liable to be fracturedby filtration or pressure.

The preferred mean pore size in the surface of the porous membranedepends on use. However, in the present invention, it is preferable thatat least one surface of the porous membrane have a mean pore size of 0.5μm or less, and more preferably 0.2 μm or less. The lower limit of themean pore size in the surface also depends on use, but it is generallypreferable to be 0.001 μm (1 nm) or more. Preferably, the pore sizedistribution is narrow. In particular, in a separation membrane forwater treatment, the mean pore size of the porous membrane surface ispreferably in the range of 0.005 to 0.5 μm, and more preferably in therange of 0.007 to 0.2 μm. A mean pore size in these ranges leads to bothhigh rejection properties and a high water permeability. Also, when themean pore size is in these ranges, the pores are not easily clogged withwater contaminants and, therefore, the water permeability is notnegatively affected. The porous membrane can, therefore, be usedsuccessively for a long time. If clogging occurs, contaminants caneasily be removed by backwash or air cleaning. The contaminants aredifferent from one water source to another. For example, in the case ofa river or a lake, the contaminants include inorganic substances derivedfrom soil and mud and their colloids, microorganisms and their corpses,and humic substances derived from plants and microorganisms. Thebackwash is performed by delivering permeate in a direction reverse to anormal filtration. The air cleaning is performed on a hollow fibermembrane by delivering air to vibrate the hollow fiber membrane so thatthe contaminants deposited on the surface of the membrane are removed.

Either surface of the membrane may be set to a mean pore size of 0.5 μmor less. In separation membranes for water treatment, it is preferablethat the external surface side, which comes in contact with water, havea three-dimensional network structure whose mean pore size is 0.5 μm orless in the external surface, and that the internal surface side have aspherical structure, from the viewpoint of the enhancement of strength,water permeability, rejection properties, and contamination resistance.

In order to determine the mean pore size in the surface, the surface ofthe porous membrane is photographed through a SEM or the like at amagnification allowing the pores to be clearly observed. The diametersof arbitrary 10 or more pores, preferably arbitrary 20 or more pores,are measured and number-averaged. Also, the mean pore size may bedetermined using an image processing system in which the mean diametersof the pores are obtained. In this instance, the mean diameter ofequivalent rounds is defined as the mean pore size. The mean diameter ofequivalent rounds is determined by the expression (a×b)^(0.5), wherein aand b are the breadth and the length of elliptical pores, respectively.

The resin used in the present invention is not particularly limited,but, preferably, a thermoplastic resin is used because it is easy toform a spherical structure. The thermoplastic resin is constituted of achain macromolecular compound, and is deformed or fluidized by anexternal force when heated. Exemplary thermoplastic resins includepolyethylene, polypropylene, acrylic resins, polyacrylonitrile,acrylonitrile-butadiene-styrene (ABS) resins, polystyrene,acrylonitrile-styrene (AS) resins, vinyl chloride resins, polyethyleneterephthalate, polyamide, polyacetal, polycarbonate, modifiedpolyphenylene ether, polyphenylene sulfide, polyvinylidene fluoride,polyamide-imide, polyetherimide, polysulfone, polyether sulfone, andtheir mixtures and copolymers.

These resins may be mixed with another resin capable of being blended.

Particularly preferably, a resin selected from the group consisting ofpolyethylene resins, polypropylene resins, and polyvinylidene fluorideresins is used as the thermoplastic resin in the present inventionbecause they have high chemical resistance.

The polyethylene resins used in the present invention contain apolyethylene homopolymer and/or a polyethylene copolymer. A plurality oftypes of polyethylene copolymer may be contained. An exemplarypolyethylene copolymer may comprise ethylene and at least onestraight-chain unsaturated hydrocarbon selected from propylene, butene,pentene, and the like.

The polypropylene resins used in the present invention contain apolypropylene homopolymer and/or a polypropylene copolymer. A pluralityof types of polypropylene copolymer may be contained. An exemplarypolyethylene copolymer may comprise propylene and at least onestraight-chain unsaturated hydrocarbon selected from ethylene, butene,pentene, and the like.

The polyvinylidene fluoride resins used in the present invention containa vinylidene fluoride homopolymer and/or a vinylidene fluoridecopolymer. A plurality of types of vinylidene fluoride copolymer may becontained. An exemplary vinylidene fluoride copolymer may comprise avinylidene fluoride and at least one selected from the group consistingof vinyl fluoride, ethylene tetrafluoride, propylene hexafluoride,chlorotrifluoroethylene. The weight-average molecular weight of thepolyvinylidene fluoride resin is appropriately selected according to thestrength and water permeability of the porous membrane required.Preferably, it is in the range of fifty thousand to one million. If theworkability of the porous membrane is taken into account, theweight-average molecular weight is preferably in the range of 100thousand to 700 thousand, and more preferably in the range of 150thousand to 600 thousand.

The polyethylene, polypropylene, and polyvinylidene fluoride resins maycontain 50 percent by weight or less of another resin capable of beingblended. For example, the polyvinylidene fluoride resin preferablycontains 50 percent by weight or less of an acrylic resin. The acrylicresin refers to a macromolecular compound mainly containing a polymer ofacrylic acid, a methacrylic acid, or their derivative, such asacrylamide or acrylonitrile. Particularly preferably, acrylic esterresins and methacrylic ester resins are used because they are misciblewith polyvinylidene fluoride resins. By preparing a polymer blendcontaining such a plurality of resins, the strength, water permeability,rejection properties, and other properties can be controlled.

The three-dimensional network structure and the spherical structure maybe formed of an identical type of resin or different types of resin.When the three-dimensional network structure and the spherical structureare formed of an identical type of resin, both structures advantageouslyhave an affinity for each other. On the other hand, when each structureis formed of a different type of resin, the strength, waterpermeability, rejection properties, and other properties can be set in awider range. The spherical structure is preferably formed of athermoplastic resin, as described above. However, the three-dimensionalnetwork structure may be formed of a resins selected from among varioustypes of resin with a wide range of choice. Either the three-dimensionalnetwork structure or the spherical structure or both may advantageouslybe formed of a polymer blend containing the same resin as in each other.By using such a polymer blend, the strength, water permeability,rejection properties, and other properties can be controlled in a widerange with a high affinity maintained between both structures.

The above-described porous membrane is used as a porous membrane modulewhich is housed in a case having a raw water inlet, a permeate outlet,and the like. In the case where the porous membrane is in a hollow fiberform, the porous membrane module is fabricated so as to recoverpermeate, by housing a bundle of plurality of hollow fiber membranes ina cylindrical container with both ends or one end fixed with apolyurethane or epoxy resin, or by fixing both ends of the hollow fibermembranes in a plate form. In the case where the porous membrane is in aflat form, the porous membrane module is fabricated so as to recoverpermeate, by folding the porous membrane in a cylindrical manner, arounda liquid collection pipe and taking up the membrane in a spiral mannerto be housed in a cylindrical container, or by disposing the membrane onboth surfaces of a liquid collection board with the periphery fixedwater tightly.

The porous membrane module is provided with at least compression meansat the raw water side or suction means at the permeate side to be usedas a liquid separation system. As the compression means, a pump may beused, or pressure caused by the difference of water levels may be used.A pump or a siphon may be used as the suction means.

This liquid separation system can be used for water purification, cleanwater treatment, effluent treatment, industrial water production, andthe like in the field of water treatment.

Thus, river water, lake water, groundwater, seawater, sewage, dischargedwater, and the like are treated.

The above-described porous membrane can also be used for a batteryseparator for separating the positive electrode and the negativeelectrode in a battery. In this instance, it is expected to enhancebattery performance because of a high ion permeability and to enhancethe durability of the battery because of a high fracture strength.

Furthermore, the porous membrane prepared by the above-describedmanufacturing method is used as a charged membrane by introducingcharged groups (ion exchange groups). The charged membrane is expectedto achieve the effects of improving ion recognition properties andenhancing the durability because of a high fracture strength.

Also, the porous membrane is used as an ion exchange membrane for a fuelcell by impregnating the porous membrane with an ion exchange resin. Inparticular, when methanol is used as a fuel, the swelling of the ionexchange membrane with methanol is suppressed and the methanol isprevented from leaking from the anode to the cathode through the ionexchange membrane, that is, so-called crossover is prevented. Thus, itis expected that the performance of the fuel cell is enhanced.Furthermore, it is expected to enhance the durability of the fuel cellbecause of a high fracture strength.

The above-described porous membrane is also used as a blood purificationmembrane. This blood purification membrane is expected to increase theproperties of removing waste products in blood, and to enhance thedurability of the blood purification membrane because of a high fracturestrength.

The porous membrane having both a three-dimensional network structureand a spherical structure can be manufactured by various techniques. Forexample, both the three-dimensional network structure and the sphericalstructure may be simultaneously formed of an identical resin solution.The three-dimensional network structure may be layered on at least onesurface of a porous membrane having the spherical structure.Alternatively, at least two types of resin solutions may be dischargedfrom an extrusion head at one time to form the three-dimensional networkstructure and the spherical structure simultaneously.

A method for simultaneously forming a three-dimensional networkstructure and a spherical structure of an identical resin solution willbe described below. In this method, for example, a thermoplastic resinis dissolved in a poor or good solvent for the resin to be a relativelyhigh concentration in the range of about 20 to 60 percent by weight.This thermoplastic resin solution is cooled to solidify. Thus, athree-dimensional network structure and a spherical structure aresimultaneously formed. The poor solvent refers to a solvent incapable ofdissolving 5 percent by weight or more of resin at a low temperature of60° C. or less, but capable of dissolving 5 percent by weight or more ofrein at a high temperature between 60° C. and the melting point of theresin (for example, about 178° C. when the resin comprises a vinylidenefluoride homopolymer). On the other hand, a solvent capable ofdissolving 5 percent by weight or more of resin at a temperature as lowas 60° C. or less is defined as a good solvent. A solvent not allowing aresin to dissolve or swell is defined as a nonsolvent. When apolyvinylidene fluoride resin is used, exemplary poor solvents includealkyl ketones, esters, glycol esters and organic carbonates having amedium chain length; such as cyclohexanone, isophorone, γ-butyrolactone,methyl isoamyl ketone, dimethyl phthalate, propylene glycol methylether, propylene carbonate, diacetone alcohol, and glycerol triacetate;and their mixtures. A mixture of a nonsolvent and a poor solvent,satisfying the above-described definition is defined as a poor solvent.Exemplary good solvents include lower alkyl ketones, such asN-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylacetamide,dimethylformamide, methyl ethyl ketone, acetone, tetrahydrofuran,tetramethylurea, and trimethyl phosphate; esters; amides; and theirmixtures. Exemplary nonsolvents include water, hexane, pentane, benzene,toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene,trichloroethylene, ethylene glycol, diethylene glycol, triethyleneglycol, propylene glycol, butylene glycol, pentanediol, hexanediol,aliphatic hydrocarbons such as low-molecular-weight polyethyleneglycols, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromaticpolyhydric alcohols, chlorinated hydrocarbons, other chlorinated organicliquids, and their mixtures.

In the above-described method, preferably, a thermoplastic resin isdissolved in a poor or good solvent for the resin at a relatively hightemperature in the range of 80 to 170° C. to prepare a thermoplasticresin solution having a relatively high concentration in the range of 20to 60 percent by weight. As the resin concentration increases, theresulting porous membrane exhibits higher stretch properties. However,an excessively high concentration reduces the void ratio and the waterpermeability is negatively affected. Unless the viscosity of theprepared resin solution is set in a proper range, the resulting membranedoes not become porous. More preferably, the resin concentration is inthe range of 30 to 50 percent by weight.

The solidification of the resin solution is, preferably, performed bycooling solidification in which the resin solution is discharged into acool bath from an extrusion head. In this instance, preferably, a liquidwith a temperature in the range of 5 to 50° C., containing 60 to 100percent of a poor or good solvent is used as a cooling liquid in thecool bath. The cooling liquid may contain a nonsolvent in addition tothe poor or good solvent. By dissolving a relatively high concentrationof a thermoplastic resin in a poor or good solvent for the resin at arelatively high temperature and rapidly cooling it to solidify, theresulting membrane can have a fine spherical structure or a densenetwork structure without macro voids. In particular, the membranehaving the spherical structure exhibits a high strength and a high waterpermeability. Whether the membrane has a spherical structure or anetwork structure is set by selecting a combination of the concentrationof the resin solution, the composition of the solvent for dissolving theresin, and the temperature of the cooling liquid in the cool bath. Onthe other hand, in the known wet solution method, since theconcentration of the resin solution is set in the range of 10 to 20percent by weight so as to achieve a water permeability, the resultingmembrane has a network structure with macro voids and does not exhibit ahigh stretch.

In order to obtain the porous membrane of the present invention, thecombination of the composition of the solvent for dissolving the resinand the composition of the cooling liquid in the cool bath is important,as well as the concentration of the resin solution. In particular, inorder to provide both a three-dimensional network structure and aspherical structure, it is preferable that the resin solution applied onone surface of the membrane be solidified using a cooling liquid havinga different composition from the composition for the other surface.Specifically, the combinations of the composition of the resin solutionand the composition of the cooling liquid are adjusted so as to form athree-dimensional network structure on one surface and form a sphericalstructure on the other surface.

If the porous membrane is formed in a hollow fiber, after a resinsolution is prepared, the resin solution and a lumen forming fluid arerespectively discharged from the external pipe and the internal pipe ofa double co-extrusion head for spinning hollow fiber membranes, whilebeing solidified in a cool bath. Thus, a hollow fiber membrane isformed. In this instance, a gas or a liquid may be used as the lumenforming fluid. However, in the present invention, the same liquid as thecooling liquid is preferably used, which contains 60 to 100 percent of apoor solvent or good solvent. In this instance, by varying thecompositions of the lumen forming fluid and the cooling liquid in thecool bath, a hollow fiber membrane having both the three-dimensionalnetwork structure and the spherical structure can be provided. The lumenforming fluid may be supplied with cooling. However, if the cool bathhas sufficient power to solidify the hollow fiber membrane, the lumenforming fluid may be supplied without cooling.

If the porous membrane is formed in a flat membrane, after a resinsolution is prepared, the resin solution is discharged from a slitextrusion head and solidified in a cool bath. In this instance, byadjusting the compositions of cooling liquids coming into contact withone surface of the flat membrane and with the other surface, or bybringing the cool bath into contact with only one surface of the flatmembrane, the resulting flat membrane can have both a three-dimensionalnetwork structure and a spherical structure. The method for varying thecooling liquid compositions coming into contact with one surface of theflat membrane and with the other surface is not particularly limited.However, for example, a cooling liquid is sprayed from one side of theflat membrane and another cooling liquid is sprayed from the other side.The method for bringing only one side of the flat membrane into contactwith a cool bath is not particularly limited. However, for example, theflat membrane may be floated on the surface of the cool bath, or acooling liquid may be sprayed from only one side of the flat membrane.

Also, it is a preferred form of embodiments of the invention that aporous substrate may further be bonded so as to support the porousmembrane to give a strength to the membrane, because the fracturestrength is enhanced. The material of the porous substrate is notparticularly limited, and organic materials and inorganic materials maybe used. However, organic materials are preferable from the viewpoint ofweight saving. More preferably, a woven or nonwoven textile comprising aorganic fiber, such as a cellulose fiber, a cellulose triacetate fiber,a polyester fiber, a polypropylene fiber, and a polyethylene fiber, maybe used.

The manufacturing method up to this point can provide a water-permeableporous membrane having high stretch properties. However, if the waterpermeability is not satisfactory, the porous membrane may further bedrawn at a draw ratio in the range of 1.1 to 5.0. This is a preferredform of embodiments of the invention because the water permeability ofthe porous membrane is enhanced.

Another method for forming a porous membrane having a three-dimensionalnetwork structure and a spherical structure will now be described. Inthis method, a layer having a three-dimensional network structure isformed afterward on at least one surface of a porous membrane having aspherical structure.

In this method, first, a porous membrane having a spherical structure isformed. The method for forming the porous membrane having the sphericalstructure is not particularly limited, but, preferably, the foregoingmethod may be applied.

A layer having a three-dimensional network structure is formed on atlest one surface of the resulting porous membrane having the sphericalstructure. The method for this is not particularly limited, but,preferably, the following method is applied. Specifically, after a resinsolution is applied onto at least one surface of the porous membranehaving the spherical structure, the membrane is immersed in asolidification liquid. Thus, a layer having a three-dimensional networkstructure is formed.

Exemplary resins used here include acrylic resins, polyacrylonitrileresins, acrylonitrile-butadiene-styrene (ABS) resins, polystyreneresins, acrylonitrile-styrene (AC) resins, vinyl chloride resins,polyethylene terephthalate resins, polyamide resins, polyacetal resins,polycarbonate resins, modified polyphenylene ether resins, polyphenylenesulfide resins, polyvinylidene fluoride resins, polyamide-imide resins,polyetherimide resins, polysulfone resins, polyether sulfone resins, andtheir mixtures and copolymers. These resins may contain another resincapable of being blended, a polyhydric alcohol, or a surfactant in anamount of 50 percent by weight or less.

Particularly preferably, a resin selected from the group consisting ofpolysulfone resins, polyether sulfone resins, acrylic resins,polyacrylonitrile resins and polyvinylidene fluoride resins is usedbecause they are chemical resistant.

A good solvent for the resin is preferably used as a solvent fordissolving the resin. As the good solvent, the foregoing substances maybe used. The concentration of the resin is generally preferably in therange of 5 to 30 percent by weight, and more preferably in the range of10 to 25 percent by weight. If the resin concentration is less than 5percent by weight, the physical durability of the layer having thethree-dimensional network structure is negatively affected. If the resinconcentration is more than 30 percent by weight, a high pressure isrequired when a liquid is allowed to permeate.

The method for applying the resin solution onto at least one surface ofthe porous membrane having the spherical structure is not particularlylimited. However, preferably, the porous membrane is immersed in thesolution, or the solution is applied onto at least one surface of theporous membrane. If the porous membrane is formed in a hollow fiber, inorder to apply the solution onto the external surface of the hollowfiber membrane, the hollow fiber membrane may be immersed in thesolution or the solution may be dripped onto the hollow fiber membrane.Also, in order to apply the solution onto the internal surface of thehollow fiber membrane, preferably, the solution is injected into thehollow fiber membrane. In order to control the amount of the solutionapplied, after the porous membrane is immersed in the solution or thesolution is applied onto the porous membrane, part of the appliedsolution may be scraped off or blown off with an air knife, in additionto the method of controlling the solution amount itself.

Preferably, a solidification liquid contains a nonsolvent for the resin.As the nonsolvent, the foregoing substances may be used. By bring theresin solution into contact with the nonsolvent, nonsolvent inducedphase separation occurs to form a layer having a three-dimensionalnetwork structure.

The method for setting the mean pore size in the surface in theforegoing range depends on the type of resin, but, for example, thefollowing method may be applied. An additive is added to the resinsolution. The additive is eluted when or after the three-dimensionalnetwork structure is formed. Thus, the mean pore size in the surface iscontrolled.

Organic compounds or inorganic compounds may be used as the additive.Preferably, the organic compounds are capable of dissolving in both thesolvent for the resin and the nonsolvent causing nonsolvent inducedphase separation. Exemplary organic compounds include water-solublepolymer, such as polyvinylpyrrolidone, polyethylene glycol, polyvinylalcohol, polyethyleneimine, polyacrylic acid, and dextran; surfactants;glycerine; and saccharides. Preferably, the inorganic compounds aresoluble in water.

Exemplary inorganic compounds include calcium chloride, lithiumchloride, and barium sulfate. Alternatively, the mean pore size in thesurface is controlled by selecting the type, concentration, andtemperature of the nonsolvent in the solidification liquid so as toadjust the phase separation speed, instead of using the additive. Ingeneral, a high phase separation speed leads to a small mean pore sizein the surface, and a low phase separation speed leads to a large meanpore size. Also, it is advantageous to add a nonsolvent to the resinsolution for controlling the phase separation speed.

Still another method for forming a porous membrane having both athree-dimensional network structure and a spherical structure will nowbe described. In this method, two or more types of resin solution aresimultaneously discharged from an extrusion head to form athree-dimensional network structure and a spherical structure at onetime. In this method, the three-dimensional network structure and thespherical structure are formed, for example, by discharging a resinsolution for forming the three-dimensional network structure and a resinsolution for forming the spherical structure and subsequentlysolidifying the solutions. Since this method allows simultaneousformation of the three-dimensional network structure and the sphericalstructure, manufacturing processes are advantageously simplified. Theresin solution for forming the three-dimensional network structure isnot particularly limited as long as it is solidified to result in athree-dimensional network structure. For example, a solution may be usedwhich is prepared by dissolving a resin in a solvent and in whichnonsolvent induced phase separation is caused by coming into contactwith a solidification bath. The resin solution for forming the sphericalstructure is not particularly limited as long as it is solidified toresult in a spherical structure. For example, a solution may be usedwhich is prepared by dissolving a thermoplastic resin, such as apolyvinylidene fluoride resin, in a relatively high concentration ofabout 20 to 60 percent by weight, in a poor or good solvent for theresin at a relatively high temperature (about 80 to 170° C.). As thethermoplastic resin, solidification bath, and poor or good solvent,preferably, the foregoing substances are used.

The extrusion head for simultaneously discharging the resins solutionfor forming the three-dimensional network structure and the sphericalstructure is not particularly limited. However, if the porous membraneis formed in a flat membrane, a double-slit head having two slits ispreferably used. If the porous membrane is formed in a hollow fibermembrane, a triple co-extrusion head is preferably used. The resins forforming the three-dimensional network structure and the sphericalstructure are discharged from the external pipe and the middle pipe ofthe triple co-extrusion head and a lumen forming fluid is dischargedfrom the internal pipe, while they are solidified in a cool bath. Thus,a hollow fiber membrane is formed. According to this method, formanufacturing a hollow fiber membrane, the amount of lumen forming fluidcan advantageously be set to be smaller than that of the cooling liquidused for forming a flat membrane. By discharging the resin for forming athree-dimensional network structure and the resin for forming aspherical structure from the external pipe and the middle pipe,respectively, a hollow fiber membrane can be obtained which has athree-dimensional network structure on the external side and a sphericalstructure on the internal side. In contrast, by discharging the resinfor forming the three-dimensional network structure and the resin forforming the spherical structure from the middle pipe and the externalpipe, respectively, a hollow fiber membrane can be obtained which hasthe three-dimensional network structure on the internal side and thespherical structure on the external side.

The above-described porous membrane is used as a porous membrane modulewhich is housed in a case having a raw water inlet, a permeate outlet,and the like. A porous membrane formed in a hollow fiber membrane isuses as a hollow fiber membrane module.

FIG. 9 shows an example of the hollow fiber membrane module. Severalhundreds to tens of thousands of hollow fiber membranes 2 are tied in abundle to prepare a hollow fiber bundle 3. The hollow fiber bundle 3 ishoused in a cylindrical case 1. Both ends of the cylindrical case 1 aresealed with sealants 3A and 3B secured to the internal walls of thecylindrical case 1. A filtration chamber 4 is provided in the spacebetween the sealants 3A and 3B in the cylindrical case. The hollow fiberbundle 3 is placed in the filtration chamber 4.

The hollow fiber bundle 3 may be disposed in a U shape in thecylindrical case 1. However, in the present invention, it is disposed inline with being fixed by the sealants and each lumen of the hollow fibermembranes 2 is open to the external surface of one or both of thesealants.

The sealants 3A and 3B are formed by injecting a fluid resin into theinterstices between the hollow fiber membranes constituting the hollowfiber bundle and, subsequently, solidifying the resin. The solidifiedresin, constituting the sealants 3A and 3B, is integrated with thehollow fiber membranes 3 and further integrated with the internal wallsof the cylindrical case 1 (such a sealing manner with a resin isreferred to as potting).

When potting is performed, the portions corresponding to the ends of theopenings of the completed hollow fiber membranes 2, acting as a finalproduct, are filled with a resin or crashed to close the openings inadvance so that the potting resin is prevented from permeating. Afterpotting, part of the sealants formed by potting is cut away such thatthe previously filled portions of the lumens are removed.

In this drawing, raw water is supplied to the filtration chamber underpressure. The raw water reaches the lumens of the hollow fiber membranesthrough the hollow fiber membranes in the filtration chamber. Duringthis flow, the raw water is filtrated to be permeate. The permeate isdischarged from the openings of the hollow fiber membranes. The hollowfiber membrane module formed as in above is provided with at leastcompression means at the raw water side or suction means at the permeateside. FIG. 10 shows an example of a water separation apparatus using ahollow fiber membrane module. The raw water stored in a raw water tank 1is pressurized with a booster pump 2 and is subsequently supplied to themembrane module 3. The pressure of the supplied raw water is measuredwith a pressure gauge 4, if necessary. The raw water is separated intopermeate and concentrated water through the membrane module 3. In theapparatus in the drawing, the concentrated water is drained outside andthe permeate is stored in a treated water tank 5. In the apparatus inthe drawing, a backwash water tank 7 is provided. By delivering water ina direction reverse to a normal direction, the membrane module 3 iscleaned. This water flow is controlled by valves 6 a to 6 d. As thecompression means, pressure caused by the difference of water levels maybe used instead of the pump. A pump or a siphon may be used as thesuction means. On the other hand, a flat porous membrane is used as aspiral element or a plate-and-frame element. These elements are alsoprovided with at least compression means at the raw water side orsuction means at the permeate side. As the compression means, a pump maybe used, or pressure caused by the difference of water levels may beused. A pump or a siphon may be used as the suction means.

FIG. 11 shows an example of a spiral element. In a spiral element 15, aporous membrane 18 formed in a bag-like manner with a feed water spacer17 wrapped therein is wound on a center pipe 16 in a spiral manner witha permeate spacer 19 in gaps between turns of the membrane. A brine seal20 is provided at one end of the spiral element. The spiral element 15guides water supplied, under a prescribed pressure, from the brine seal20 side to the porous membrane 18 through the permeate spacer 19.Permeate which has passed through the porous membrane 18 is taken outthrough the center pipe 16.

Such an element having this structure can have a membrane area largerthan that of a plate-and-frame element, described later, and,accordingly the amount of permeate can advantageously be set large.However, the spiral element is rather vulnerable to contamination. Itis, therefore, suitable for clean raw water (such as clean sea water,brine water, and river water).

FIG. 12 shows an example of a plate-and-frame element. A permeate spacer28 and a porous membrane 29 are disposed in that order on both surfacesof a supporting plate 27 having a high stiffness and the periphery iswater-tightly fixed. Each surface of the supporting plate 27 hasprotuberances and a recess. The porous membrane 29 filters outimpunities in water. The permeate spacer 28 is intended to efficientlydeliver the permeate filtered through the porous membrane 29 to thesupporting plate 27. The permeate flowing to the supporting plate 27 istaken out outside through the recesses of the supporting plate 27. Thepermeate spacer 28 and the porous membrane 29 are disposed at only onesurface side of the supporting plate 27. However, by disposing them atboth surfaces, the membrane area can be increased.

In such an element having the above-described structure, the permeatefiltered through the porous membrane 29 passes through the permeatespacer 28 and the recess of the supporting plate 27, and is finallytaken out of a permeate outlet 30 to the outside of the element.

A porous membrane module is such that a plurality of above-describedelements are disposed, in a housing, parallel to one another such as toform spaces between the surfaces thereof.

A fuel cell membrane will now be described with reference to a drawing.FIG. 13 is a schematic illustration of an MEA (membrane electrodeassembly) of a direct methanol fuel cell. An electrolyte 31 is disposedbetween an anode 32 and a cathode 30. By supplying methanol, acting as afuel, to the anode side, an electromotive force is generated. The poresof the porous membrane of the present invention is impregnated with anelectrolyte, such as a polymer electrolyte. Thus, the porous membrane isused as the electrolyte membrane 31.

The present invention will further be described using concrete examples.However, the present invention is not limited by these examples.

In order to determine the mean pore size of porous membranes and themean diameter of the spherical structure in examples, a cross section ofeach porous membrane was photographed at a magnification of 1,000 or10,000 through an SEM (S-800) (manufactured by Hitachi, Ltd.). The poresizes and the diameters in the spherical structure of arbitrary 10 to 50pores were measured and number-averaged. In order to determine the meanpore size in the surface of a porous membrane, a cross section of theporous membrane was photographed at a magnification of 1,000 or 10,000through the above-mentioned SEM. The pore sizes of arbitrary 10 to 50pores were measured and number-averaged.

The measurements of the water permeability and rejection property of thehollow fiber porous membrane were performed on a miniature module of 200mm in length including four hollow fiber membranes. In the case of theflat porous membrane, the measurements were performed on a membranewhich is cut to a circle of 50 mm in diameter and set in a cylindricalfiltration holder. Reverse osmosis membrane treated water was entirelyfiltered for 30 minutes by external pressure at a temperature of 25° C.and a differential pressure of 16 kPa. The quantity of permeate (m³) wasconverted into a value per hour (h) and a value per effective membranearea (m²). These values were further multiplied by 50/16 and convertedinto a value at a pressure of 50 kPa. Thus, the water permeability wasdetermined. Water in which polystyrene latex particles having a meanparticle size of 0.843 μm were dispersed was entirely filtered for 30minutes by external pressure at a temperature of 25° C. and adifferential pressure of 16 kPa. The rejection property were determinedfrom the ratio of the latex particle concentration in raw water to thatin permeate. These latex particle concentrations are obtained bymeasuring absorption coefficients of ultraviolet light having awavelength of 240 nm. The absorption coefficients of ultraviolet lighthaving a wavelength of 240 nm were measured with a spectrophotometer(U-3200) (manufactured by Hitachi, Ltd.).

The fracture strength and the fracture elongation were determined by atensile test using a tensile tester (TENSILON/RTM-100) (manufactured byToyo Baldwin). The tensile test was performed on five samples having ameasurement length of 50 mm at a tensile speed of 50 mm/min, andobtained fracture strengths and fracture elongations were averaged.

Example 1

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand in an amount of 38 percent by weight was mixedwith 31 percent by weight of γ-butyrolactone and 31 percent by weight ofdiethylene glycol and dissolved at a temperature of 200° C. This resinsolution was discharged with γ-butyrolactone, acting as a lumen formingfluid, from a double co-extrusion head with a temperature of 190° C.,and was solidified in a bath with a temperature of 12° C. containing anaqueous solution of 80 percent by weight γ-butyrolactone. The resultinghollow fiber membrane had an outer diameter of 1.60 mm and an innerdiameter of 0.90 mm. FIG. 1 shows an SEM photograph of an entire sectionof the hollow fiber membrane; FIG. 2 shows an SEM photograph of asection around the external surface; and FIG. 3 shows an SEM photographof a section around the internal surface. The area around the externalsurface had a three-dimensional network structure and the area aroundthe internal surface had a spherical structure. Hence, it has been shownthat the three-dimensional network structure and the spherical structurecoexist. The mean pore size of the three-dimensional network structurewas 1.63 μm, and the mean diameter of the spherical structure was 4.06μm. The water permeability at 50 kPa and 25° C. was 0.30 m³/m²·h. Therejection for particles having a particle size of 0.843 μm was 98%. Thefracture strength and the fracture elongation were 7.8 MPa and 104%,respectively. Thus, the hollow fiber membrane had a dense surface andexhibited excellent water permeability, rejection properties, strength,and elongation.

Example 2

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand in an amount of 38 percent by weight was mixedwith 31 percent by weight of γ-butyrolactone and 31 percent by weight ofdiethylene glycol and dissolved at a temperature of 200° C. This resinsolution was discharged with γ-butyrolactone, acting as a lumen formingfluid, from a double co-extrusion head with a temperature of 180° C.,and was solidified in a bath with a temperature of 17° C. containing anaqueous solution of 80 percent by weight γ-butyrolactone. The resultinghollow fiber membrane had an outer diameter of 1.50 mm and an innerdiameter of 0.92 mm. The area around the external surface had athree-dimensional network structure and the area around the internalsurface had a spherical structure. Hence, it has been shown that thethree-dimensional network structure and the spherical structure coexist.The mean pore size of the three-dimensional network structure was 1.73μm, and the mean diameter of the spherical structure was 4.89 μm. Thewater permeability at 50 kPa and 25° C. was 0.35 m³/m²·h. The rejectionfor particles having a particle size of 0.843 μm was 97%. The fracturestrength and the fracture elongation were 8.8 MPa and 100%,respectively. Thus, the hollow fiber membrane had a dense surface andexhibited excellent water permeability, rejection properties, strength,and elongation.

Example 3

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand in an amount of 38 percent by weight was mixedwith 41 percent by weight of γ-butyrolactone and 21 percent by weight ofdiethylene glycol and dissolved at a temperature of 200° C. This resinsolution was discharged with γ-butyrolactone, acting as a lumen formingfluid, from a double co-extrusion head with a temperature of 160° C.,and was solidified in a bath with a temperature of 12° C. containing anaqueous solution of 80 percent by weight γ-butyrolactone. The resultinghollow fiber membrane had an outer diameter of 1.54 mm and an innerdiameter of 0.93 mm. The area around the external surface had athree-dimensional network structure and the area around the internalsurface had a spherical structure. Hence, it has been shown that thethree-dimensional network structure and the spherical structure coexist.The mean pore size of the three-dimensional network structure was 1.26μm, and the mean diameter of the spherical structure was 2.61 μm. Thewater permeability at 50 kPa and 25° C. was 0.20 m³/m²·h. The rejectionfor particles having a particle size of 0.843 μm was 99%. The fracturestrength and the fracture elongation were 5.5 MPa and 99%, respectively.Thus, the hollow fiber membrane had a dense surface and exhibitedexcellent water permeability, rejection properties, strength, andelongation.

Example 4

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand in an amount of 38 percent by weight was mixedwith 41 percent by weight of γ-butyrolactone and 21 percent by weight ofdiethylene glycol and dissolved at a temperature of 200° C. This resinsolution was discharged with γ-butyrolactone, acting as a lumen formingfluid, from a double co-extrusion head with a temperature of 150° C.,and was solidified in a bath with a temperature of 14° C. containing anaqueous solution of 80 percent by weight γ-butyrolactone. The resultinghollow fiber membrane had an outer diameter of 1.56 mm and an innerdiameter of 0.98 mm. The area around the external surface had athree-dimensional network structure and the area around the internalsurface had a spherical structure. Hence, it has been shown that thethree-dimensional network structure and the spherical structure coexist.The mean pore size of the three-dimensional network structure was 1.22μm, and the mean diameter of the spherical structure was 3.51 μm. Thewater permeability at 50 kPa and 25° C. was 0.25 m³/m²·h. The rejectionfor particles having a particle size of 0.843 μm was 98%. The fracturestrength and the fracture elongation were 6.0 MPa and 23%, respectively.Thus, the hollow fiber membrane had a dense surface and exhibitedexcellent water permeability, rejection properties, strength, andelongation.

Example 5

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand in an amount of 38 percent by weight was mixedwith 31 percent by weight of γ-butyrolactone and 31 percent by weight ofdiethylene glycol and dissolved at a temperature of 200° C. This resinsolution was discharged from a slit head with a temperature of 190° C.γ-butyrolactone with a temperature of 12° C. was sprayed onto on surfaceof the discharged resin and an aqueous solution of 80 percent by weightof γ-butyrolactone was sprayed onto the other surface. Thus the resinwas solidified. The resulting flat membrane had a thickness of 0.175 mm.One surface of the resin had a three-dimensional network structure andthe other surface had a spherical structure. Hence, it has been shownthat the three-dimensional network structure and the spherical structurecoexist. The mean pore size of the three-dimensional network structurewas 1.60 μm, and the mean diameter of the spherical structure was 4.10μm. The water permeability at 50 kPa and 25° C. was 0.25 m³/m²·h. Therejection for particles having a particle size of 0.843 μm was 98%. Thefracture strength and the fracture elongation were 7.5 MPa and 40%,respectively. Thus, the flat membrane had a dense surface and exhibitedexcellent water permeability, rejection properties, strength, andelongation.

Comparative Example 1

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand was mixed with dimethyl sulfoxide at a ratio of20 to 80 percent by weight, respectively, and was dissolved at atemperature of 70° C. This resin solution was discharged with an aqueoussolution of 50 percent by weight dimethyl sulfoxide, acting as a lumenforming fluid, from a double co-extrusion head with a temperature of 50°C., and was solidified in a bath with a temperature of 50° C. containingan aqueous solution of 50 percent by weight dimethyl sulfoxide. Theresulting hollow fiber membrane had an outer diameter of 1.40 mm and aninner diameter of 0.98 mm. The external surface of the resin had a denselayer and the internal surface had an asymmetrical three-dimensionalnetwork structure with macro voids. There was no spherical structure.The water permeability at 50 kPa and 25° C. was as low as 0.08 m³/m²·h.The rejection for particles having a particle size of 0.843 μm was 99%.The fracture strength and the fracture elongation were 1.0 MPa and 48%,respectively. Thus, the strength was low.

Comparative Example 2

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand was mixed with γ-butyrolactone at a ratio of 38to 62 percent by weight, respectively, and was dissolved at atemperature of 170° C. This resin solution was discharged withγ-butyrolactone, acting as a lumen forming fluid, from a doubleco-extrusion head with a temperature of 100° C., and was solidified in acool bath with a temperature of 20° C. containing an aqueous solution of80 percent by weight γ-butyrolactone. The resulting hollow fibermembrane had an outer diameter of 1.01 mm and an inner diameter of 0.72mm. FIG. 4 shows an SEM photograph of an entire section of this hollowfiber membrane; FIG. 5 shows an SEM photograph of a section around theexternal surface; and FIG. 6 shows an SEM photograph of a section aroundthe internal surface. This membrane had only a spherical structurehaving a mean diameter of 2.75 μm. The water permeability at 50 kPa and25° C. was 0.30 m³/m²·h. The rejection for particles having a particlesize of 0.843 μm was 80%. The fracture strength and the fractureelongation were 5.3 MPa and 48%, respectively. Although the hollow fibermembrane exhibited an excellent water permeability, strength, andelongation, its rejection properties were inferior.

Example 6

A catalyst-supporting carbon (catalyst: 29.2 percent by weight ofplatinum and 15.8 percent by weight of ruthenium, carbon: Vulcan(registered trade mark) XC-72 produced by Cabot) was added into a Nafion(registered trade mark) solution (bought from Aldrich) such that theweight ratio was 1 to 1. The mixture was sufficiently stirred to preparea catalyst-polymer composition. This catalyst-polymer composition wasapplied onto one surface of a carbon paper TGP-H-060, produced by Toray,to prepare an electrode base with an electrode catalyst layer,supporting 3 mg/cm² of platinum.

On the other hand, the flat membrane produced in Example 5 was immersedin a Nafion® solution (bought from Aldrich) and subsequently dried toprepare an ion exchange membrane. Two electrode bases with an electrodecatalyst layer, described above, were layered on both surfaces of theresulting ion exchange membrane.

The ion exchange membrane serves as an electrolyte membrane. Respectivetwo electrode bases serve as a cathode and an anode. The electrodesbases were disposed such that each electrode catalyst layer side of thebases opposes the ion exchange membrane. This composite was hot-pressedunder the conditions of 130° C. and 5 MPa.

Thus, a membrane-electrode integrated unit was prepared.

The resulting membrane-electrode integrated unit was incorporated into afuel cell. An aqueous solution of 64 percent by weigh methanol and airwere supplied to the anode side and the cathode side, respectively. Thefuel cell outputs a maximum power of 0.5 mW/cm² and, thus, exhibitedexcellently high power properties. This is probably because the swellingof the ion exchange membrane with methanol is suppressed and the leak ofmethanol through the ion exchange membrane from the anode to cathode,that is, crossover, is prevented.

Comparative Example 3

Electrode bases with an electrode catalyst layer, prepared under thesame conditions as in Example 6 were layered on both surfaces of an ionexchange membrane Nafion (registered trade mark) 117 (0.175 mm inthickness), produced by Du Pont. The electrode bases were disposed suchthat each electrode catalyst layer side of the bases opposes the ionexchange membrane. This composite was hot-pressed under the sameconditions as in Example 6. Thus, a membrane-electrode integrated unitwas prepared.

The resulting membrane-electrode integrated unit was evaluated under thesame conditions as in Example 6. The maximum power was as low as 0.1mW/cm². This is probably because the ion exchange membrane Nafion(registered trade mark) 117 was welled with methanol and, consequently,crossover of methanol occurred.

Examples 7 to 18 and Comparative Examples 4 to 15

Hollow fiber membranes were prepared by the following method. First,hollow fiber membranes of Comparative Examples 4 to 15 were preparedunder the conditions shown in Table 1. Vinylidene fluoride homopolymerwas mixed with a solvent to be dissolved. The solution was dischargedwith a lumen forming fluid from a double co-extrusion head andsolidified in a cool bath. Each hollow fiber membrane had only aspherical structure. Next, A mixed solution containing 13 percent byweight of vinylidene fluoride homopolymer having a weight-averagemolecular weight of 284 thousand, 5 percent by weight of polyethyleneglycol having a weight-average molecular weight of 20 thousand, 79percent by weight of dimethylformamide, and 3 percent by weight of waterwas uniformly applied onto the surfaces of the hollow fiber membranes ofComparative Examples 4 to 15. These hollow fiber membranes wereimmediately immersed in a mixed solvent containing 95 percent by weightof water and 5 percent by weight of dimethylformamide to solidify thesolution. These are defined as Examples 7 to 18. These hollow fibermembranes each had a three-dimensional network structure on the externalside and a spherical structure on the internal side. The thickness ofthe three-dimensional network structure was 20 μm. Table 2 showproperties of hollow fiber membranes of Examples 7 to 18 and ComparativeExamples 4 to 15. Each example exhibited higher rejection propertiesthan those of the comparative examples.

TABLE 1 PVDF weight- average Ratio of Dissolving Head Cool bathmolecular polymer temp. Lumen forming temp. Cool bath temp. weightSolvent to solvent (° C.) fluid composition (° C.) composition (° C.)Comparative 284,000 Cyclohexanone 55:45 160 Cyclohexanone 100% 125 90%Cyclohexanone aq 30 Example 4 Comparative 358,000 Cyclohexanone 50:50160 Cyclohexanone 100% 120 85% Cyclohexanone aq 25 Example 5 Comparative417,000 Cyclohexanone 40:60 160 Cyclohexanone 100% 130 80% Cyclohexanoneaq 20 Example 6 Comparative 417,000 Cyclohexanone 20:80 140 95%Cyclohexanone aq 95 80% Cyclohexanone aq 10 Example 7 Comparative572,000 Cyclohexanone 35:65 170 Cyclohexanone 100% 155 80% Cyclohexanoneaq 15 Example 8 Comparative 417,000 γ-butyrolactone 40:60 170γ-butyrolactone 100% 100 80% γ-butyrolactone aq 27 Example 9 Comparative417,000 γ-butyrolactone 45:55 170 γ-butyrolactone 100% 120 80%γ-butyrolactone aq 27 Example 10 Comparative 417,000 γ-butyrolactone38:62 170 γ-butyrolactone 100% 95 80% γ-butyrolactone aq 28 Example 11Comparative 417,000 γ-butyrolactone 43:57 170 γ-butyrolactone 100% 11080% γ-butyrolactone aq 28 Example 12 Comparative 417,000 γ-butyrolactone50:50 170 γ-butyrolactone 100% 140 80% γ-butyrolactone aq 27 Example 13Comparative 417,000 γ-butyrolactone 38:62 170 γ-butyrolactone 100% 10080% γ-butyrolactone aq 27 Example 14 Comparative 358,000 γ-butyrolactone50:50 170 γ-butyrolactone 100% 113 80% γ-butyrolactone aq 27 Example 15

TABLE 2 Outer Inner Water Fracture Fracture Rejection Base hollowdiameter diameter External surface permeability strength elongationproperty fiber membrane (mm) (mm) pore size (μm) (m³/m² · h · 50 kPa)(MPa) (%) (%) Example 7 Comparative 1.74 0.99 0.04 0.20 3.29 75 99Example 4 Example 8 Comparative 1.67 1.00 0.03 0.27 5.03 68 99 Example 5Example 9 Comparative 1.56 0.88 0.10 0.35 6.70 59 97 Example 6 Example10 Comparative 1.48 0.92 0.19 0.39 3.90 63 95 Example 7 Example 11Comparative 1.55 0.89 0.18 0.20 5.66 58 95 Example 8 Example 12Comparative 1.32 0.78 0.09 0.15 4.40 60 97 Example 9 Example 13Comparative 1.26 0.74 0.09 0.20 5.76 59 97 Example 10 Example 14Comparative 1.37 0.99 0.05 0.40 7.10 243 99 Example 11 Example 15Comparative 1.53 1.15 0.17 2.39 4.66 70 96 Example 12 Example 16Comparative 1.90 1.14 0.10 0.41 3.35 80 97 Example 13 Example 17Comparative 1.44 0.85 0.04 0.26 7.02 63 99 Example 14 Example 18Comparative 1.38 0.93 0.06 0.31 10.69 75 98 Example 15 Comparative —1.70 0.99 0.78 0.35 3.53 75 86 Example 4 Comparative — 1.63 1.00 0.690.455 5.43 68 87 Example 5 Comparative — 1.52 0.88 2.20 0.605 7.23 59 76Example 6 Comparative — 1.44 0.92 5.00 0.675 4.27 63 50 Example 7Comparative — 1.51 0.89 4.60 0.34 6.13 58 53 Example 8 Comparative —1.28 0.78 1.80 0.24 4.85 60 79 Example 9 Comparative — 1.22 0.74 1.700.34 6.37 59 80 Example 10 Comparative — 1.33 0.99 0.95 0.68 8.07 243 84Example 11 Comparative — 1.49 1.15 4.30 4.11 5.29 70 56 Example 12Comparative — 1.86 1.14 2.10 0.70 3.58 80 77 Example 13 Comparative —1.40 0.85 0.80 0.445 7.67 63 85 Example 14 Comparative — 1.34 0.93 1.100.535 11.94 75 83 Example 15

Examples 19 to 30 and Comparative Examples 16 to 27

Hollow fiber membranes were prepared by the following method. First,hollow fiber membranes of Comparative Examples 16 to 27 were preparedunder the conditions shown in Table 3. Vinylidene fluoride homopolymerwas mixed with a solvent to be dissolved. The solution was dischargedwith a lumen forming fluid from a double co-extrusion head andsolidified in a cool bath. Then, the product was drawn in a drawingbath. FIG. 7 shows an SEM photograph of a cross section around theexternal surface of a hollow fiber membrane prepared in ComparativeExample 23, as a typical example. Each hollow fiber membrane ofComparative Examples 16 to 27 had only a spherical structure. A mixedsolution containing 13 percent by weight of vinylidene fluoridehomopolymer having a weight-average molecular weight of 284 thousand, 5percent by weight of polyethylene glycol having a weight-averagemolecular weight of 20 thousand, 79 percent by weight ofdimethylformamide, and 3 percent by weight of water was uniformlyapplied onto the surfaces of the hollow fiber membranes of ComparativeExamples 16 to 27. These hollow fiber membranes were immediatelyimmersed in a mixed solvent containing 95 percent by weight of water and5 percent by weight of dimethylformamide to solidify the solution. Theseare defined as Examples 19 to 30. FIG. 8 shows an SEM photograph of across section around the external surface of a hollow fiber membraneprepared in Example 26, as a typical example. Each hollow fiber membraneof Examples 19 to 30 had a spherical structure on the internal side anda three-dimensional network structure on the external side. Thethickness of the three-dimensional network structure was 20 μm. Table 4show properties of hollow fiber membranes of Examples 19 to 30 andComparative Examples 16 to 27. Each example exhibited higher rejectionproperties than those of the comparative examples.

TABLE 3 PVDF weight-average Ratio of polymer Dissolving Lumen formingHead temp. molecular weight Solvent to solvent temp. (° C.) fluidcomposition (° C.) Comparative 284,000 Cyclohexanone 55:45 160Cyclohexanone 100% 125 Example 16 Comparative 358,000 Cyclohexanone50:50 160 Cyclohexanone 100% 120 Example 17 Comparative 417,000Cyclohexanone 40:60 160 Cyclohexanone 100% 130 Example 18 Comparative417,000 Cyclohexanone 20:80 140 95% Cyclohexanone aq 95 Example 19Comparative 572,000 Cyclohexanone 35:65 170 Cyclohexanone 100% 155Example 20 Comparative 417,000 γ-butyrolactone 40:60 170 γ-butyrolactone100% 100 Example 21 Comparative 417,000 γ-butyrolactone 45:55 170γ-butyrolactone 100% 120 Example 22 Comparative 417,000 γ-butyrolactone38:62 170 γ-butyrolactone 100% 95 Example 23 Comparative 417,000γ-butyrolactone 43:57 170 γ-butyrolactone 100% 110 Example 24Comparative 417,000 γ-butyrolactone 50:50 170 γ-butyrolactone 100% 140Example 25 Comparative 417,000 Isophorone 40:60 155 100% Isophorone 100Example 26 Comparative 417,000 Dimethyl sulfoxide 30:70 95 Dimethylsulfoxide 90% aq 95 Example 27 Drawing Draw Cool bath Cool bath bathtemp. ratio composition temp. (° C.) Drawing bath (° C.) (Times)Comparative 90% Cyclohexanone aq 30 Water 88 2.0 Example 16 Comparative85% Cyclohexanone aq 25 Polyethylene glycol 110 2.5 Example 17(weight-average molecular weight: 400) Comparative 80% Cyclohexanone aq20 Water 85 3.0 Example 18 Comparative 80% Cyclohexanone aq 10 Water 853.5 Example 19 Comparative 80% Cyclohexanone aq 15 Water 85 4.0 Example20 Comparative 80% γ-butyrolactone aq 27 Water 80 2.2 Example 21Comparative 80% γ-butyrolactone aq 27 Water 80 1.6 Example 22Comparative 80% γ-butyrolactone aq 28 Water 81 1.7 Example 23Comparative 80% γ-butyrolactone aq 28 Water 80 1.5 Example 24Comparative 80% γ-butyrolactone aq 27 Water 87 1.9 Example 25Comparative 80% Isophorone aq 27 Water 85 3.0 Example 26 Comparative 90%Dimethyl sulfoxide aq 27 Water 80 1.5 Example 27

TABLE 4 Outer Inner External External External Base hollow diameterdiameter surface pore surface pore surface pore fiber membrane (mm) (mm)width (μm) length (μm) size (μm) Example 19 Comparative 1.59 0.95 — —0.04 Example 16 Example 20 Comparative 1.44 0.9 — — 0.03 Example 17Example 21 Comparative 1.34 0.75 — — 0.12 Example 18 Example 22Comparative 1.24 0.7 — — 0.18 Example 19 Example 23 Comparative 1.39 0.8— — 0.19 Example 20 Example 24 Comparative 1.11 0.64 — — 0.04 Example 21Example 25 Comparative 1.20 0.68 — — 0.03 Example 22 Example 26Comparative 1.56 0.95 — — 0.04 Example 23 Example 27 Comparative 1.471.07 — — 0.12 Example 24 Example 28 Comparative 1.53 0.93 — — 0.11Example 25 Example 29 Comparative 1.44 0.9 — — 0.17 Example 26 Example30 Comparative 1.99 1.55 — — 0.05 Example 27 Comparative — 1.55 0.950.96 3.2 1.75 Example 16 Comparative — 1.4 0.9 0.65 2.4 1.25 Example 17Comparative — 1.3 0.75 1.95 9.8 4.37 Example 18 Comparative — 1.2 0.73.20 18.3 7.65 Example 19 Comparative — 1.35 0.8 2.80 26.4 8.60 Example20 Comparative — 1.07 0.64 1.10 3.0 1.82 Example 21 Comparative — 1.160.68 0.50 5.0 1.58 Example 22 Comparative — 1.52 0.95 1.30 4.2 2.34Example 23 Comparative — 1.43 1.07 3.00 6.2 4.31 Example 24 Comparative— 1.49 0.93 2.50 6.0 3.87 Example 25 Comparative — 1.4 0.9 3.30 15.17.06 Example 26 Comparative — 1.95 1.55 1.00 4.1 2.02 Example 27 WaterFracture Fracture Rejection Base hollow permeability strength elongationproperty fiber membrane (m³/m² · h · 50 kPa) (MPa) (%) (%) Example 19Comparative 0.55 6.76 55 99 Example 16 Example 20 Comparative 0.73 12.3550 99 Example 17 Example 21 Comparative 1.05 17.42 48 97 Example 18Example 22 Comparative 1.40 7.27 50 95 Example 19 Example 23 Comparative0.61 13.34 45 95 Example 20 Example 24 Comparative 0.50 7.89 46 99Example 21 Example 25 Comparative 0.99 8.81 41 99 Example 22 Example 26Comparative 1.27 12.79 189 99 Example 23 Example 27 Comparative 2.916.39 46 97 Example 24 Example 28 Comparative 0.79 6.94 56 97 Example 25Example 29 Comparative 0.82 9.98 54 96 Example 26 Example 30 Comparative1.19 4.25 32 99 Example 27 Comparative — 0.95 7.33 55 79 Example 16Comparative — 1.25 13.57 50 82 Example 17 Comparative — 1.80 19.05 48 55Example 18 Comparative — 2.40 8.02 50 25 Example 19 Comparative — 1.0514.57 45 20 Example 20 Comparative — 0.85 8.83 46 79 Example 21Comparative — 1.70 9.75 41 81 Example 22 Comparative — 2.18 14.68 189 75Example 23 Comparative — 5.00 7.21 46 56 Example 24 Comparative — 1.357.56 56 62 Example 25 Comparative — 1.40 10.97 54 30 Example 26Comparative — 2.05 4.73 32 78 Example 27

Examples 31 to 42

A solution containing 8 percent by weight of polyacrylonitrile polymerhaving a weight-average molecular weight of 400 thousand and 92 percentby weight of dimethyl sulfoxide was applied onto the surfaces ofComparative Examples 16 to 27. These hollow fiber membranes wereimmediately immersed in a mixed solvent containing 90 percent by weightof water and 10 percent by weight of dimethyl sulfoxide to solidify thesolution. These are defined as Examples 31 to 42. These hollow fibermembranes of Examples 31 to 42 each had a three-dimensional networkstructure on the external side and a spherical structure on the internalside. The thickness of the three-dimensional network structure was 20 to30 μm. Table 5 show properties of hollow fiber membranes of Examples 31to 42. Each example exhibited higher rejection properties than those ofthe comparative examples.

TABLE 5 Outer Inner External External External Base hollow diameterdiameter surface pore surface pore surface pore fiber membrane (mm) (mm)width (μm) length (μm) size (μm) Example 31 Comparative 1.60 0.95 — —0.01 Example 16 Example 32 Comparative 1.44 0.9 — — 0.02 Example 17Example 33 Comparative 1.35 0.75 — — 0.02 Example 18 Example 34Comparative 1.25 0.7 — — 0.01 Example 19 Example 35 Comparative 1.39 0.8— — 0.01 Example 20 Example 36 Comparative 1.12 0.64 — — 0.02 Example 21Example 37 Comparative 1.20 0.68 — — 0.01 Example 22 Example 38Comparative 1.57 0.95 — — 0.01 Example 23 Example 39 Comparative 1.481.07 — — 0.02 Example 24 Example 40 Comparative 1.53 0.93 — — 0.01Example 25 Example 41 Comparative 1.45 0.9 — — 0.02 Example 26 Example42 Comparative 1.99 1.55 — — 0.02 Example 27 Water Fracture FractureRejection Base hollow permeability strength elongation property fibermembrane (m³/m² · h · 50 kPa) (MPa) (%) (%) Example 31 Comparative 0.176.85 54 99 Example 16 Example 32 Comparative 0.19 12.40 53 99 Example 17Example 33 Comparative 0.25 17.02 52 99 Example 18 Example 34Comparative 0.30 6.99 53 99 Example 19 Example 35 Comparative 0.18 13.0241 99 Example 20 Example 36 Comparative 0.12 7.97 42 99 Example 21Example 37 Comparative 0.16 8.21 44 99 Example 22 Example 38 Comparative0.29 12.23 179 99 Example 23 Example 39 Comparative 0.34 6.50 50 99Example 24 Example 40 Comparative 0.18 6.58 54 99 Example 25 Example 41Comparative 0.20 9.56 59 99 Example 26 Example 42 Comparative 0.27 4.3233 99 Example 27

Examples 43 to 54

A solution containing 2 percent by weight of polysulfone having aweight-average molecular weight of 47 thousand, 6 percent by weight ofpolysulfone having a weight-average molecular weight of 59 thousand, 3percent by weight of polyvinylpyrrolidone having a weight-averagemolecular weight of 1200 thousand, 88 percent by weight ofdimethylacetamide, and 1 percent by weight of water was applied onto thesurfaces of Comparative Examples 16 to 27. These hollow fiber membraneswere immediately immersed in a solidification bath with a temperature of40° C. containing 10 percent by weight of mixed solvent containing 10percent by weight of dimethyl sulfoxide, 30 percent by weight ofdimethylacetamide, and 60 percent by weight of water to solidify thesolution. These are defined as Examples 43 to 54. These hollow fibermembranes of Examples 43 to 54 each had a three-dimensional networkstructure on the external side and a spherical structure on the internalside. The thickness of the three-dimensional network structure was about30 μm.

Table 6 show properties of hollow fiber membranes of Examples 43 to 54.Each example exhibited higher rejection properties than those of thecomparative examples.

TABLE 6 Outer Inner External External External Base hollow diameterdiameter surface pore surface pore surface pore fiber membrane (mm) (mm)width (μm) length (μm) size (μm) Example 43 Comparative 1.60 0.95 — —0.02 Example 16 Example 44 Comparative 1.44 0.9 — — 0.03 Example 17Example 45 Comparative 1.35 0.75 — — 0.02 Example 18 Example 46Comparative 1.25 0.7 — — 0.03 Example 19 Example 47 Comparative 1.39 0.8— — 0.02 Example 20 Example 48 Comparative 1.12 0.64 — — 0.03 Example 21Example 49 Comparative 1.20 0.68 — — 0.01 Example 22 Example 50Comparative 1.57 0.95 — — 0.02 Example 23 Example 51 Comparative 1.481.07 — — 0.02 Example 24 Example 52 Comparative 1.53 0.93 — — 0.01Example 25 Example 53 Comparative 1.45 0.9 — — 0.03 Example 26 Example54 Comparative 1.99 1.55 — — 0.02 Example 27 Water Fracture FractureRejection Base hollow permeability strength elongation property fibermembrane (m³/m² · h · 50 kPa) (MPa) (%) (%) Example 43 Comparative 0.206.23 56 99 Example 16 Example 44 Comparative 0.21 12.59 60 99 Example 17Example 45 Comparative 0.29 17.33 53 99 Example 18 Example 46Comparative 0.36 6.68 70 99 Example 19 Example 47 Comparative 0.32 13.2445 99 Example 20 Example 48 Comparative 0.25 8.23 46 99 Example 21Example 49 Comparative 0.26 8.50 42 99 Example 22 Example 50 Comparative0.33 12.10 180 99 Example 23 Example 51 Comparative 0.40 6.74 63 99Example 24 Example 52 Comparative 0.20 6.56 55 99 Example 25 Example 53Comparative 0.36 9.30 73 99 Example 26 Example 54 Comparative 0.40 4.1141 99 Example 27

Example 55

A vinylidene fluoride homopolymer having a weight-average molecularweight of 358 thousand, a copolymer of ethylene tetrafluoride andvinylidene fluoride (ATOFINA Japan, Kynar (registered trade mark) 7201,weight ratio: 3:7), and cyclohexanone were mixed in amounts of 30, 10,and 60 percent by weight, respectively, and the resins were dissolved at165° C. This polymer solution was discharged with 100% cyclohexanone,acting as a lumen forming fluid, from a double co-extrusion head with atemperature of 145° C., and was solidified in a cool bath with atemperature of 30° C. containing an aqueous solution of 90 percent byweight cyclohexanone. The product was drawn to 3.0 times in water with atemperature of 80° C. to obtain a hollow fiber membrane. A mixedsolution containing 13 percent by weight of vinylidene fluoridehomopolymer having a weight-average molecular weight of 284 thousand, 5percent by weight of polyethylene glycol having a weight-averagemolecular weight of 20 thousand, 79 percent by weight ofdimethylformamide, and 3 percent by weight of water was uniformlyapplied onto the surfaces of this hollow fiber membrane. The hollowfiber membrane was then immediately immersed in a mixed solventcontaining 95 percent by weight of water and 5 percent by weight ofdimethylformamide. The resulting hollow fiber membrane had athree-dimensional network structure on the external side and a sphericalstructure on the internal side. The thickness of the three-dimensionalnetwork structure was 20 μm. The hollow fiber membrane had an outerdiameter of 1.44 mm and an inner diameter of 0.90 mm. The externalsurface of the hollow fiber membrane had pores with a mean pore size of0.11 μm. The water permeability (conditions: differential pressure 50kpa, 25° C.) was 0.44 m³/m²·h. The rejection was 97%. The fracturestrength and the fracture elongation were 15.61 MPa and 55%,respectively. Thus, the hollow fiber membrane exhibited excellent waterpermeability, rejection properties, strength, and elongation.

Comparative Example 28

A vinylidene fluoride homopolymer having a weight-average molecularweight of 358 thousand, a copolymer of ethylene tetrafluoride andvinylidene fluoride (ATOFINA Japan, Kynar (registered trade mark) 7201,weight ratio: 3:7), and cyclohexanone were mixed in amounts of 30, 10,and 60 percent by weight, respectively, and the resins were dissolved at165° C. This polymer solution was discharged with 100% cyclohexanone,acting as a lumen forming fluid, from a double co-extrusion head with atemperature of 145° C., and was solidified in a cool bath with atemperature of 30° C. containing an aqueous solution of 90 percent byweight cyclohexanone. The product was drawn to 3.0 times in water with atemperature of 80° C. to obtain a hollow fiber membrane. The resultinghollow fiber membrane had only a spherical structure. This hollow fibermembrane had an outer diameter of 1.40 mm and an inner diameter of 0.90mm. The external surface of the hollow fiber membrane had pores with abreadth of 1.8 μm, a length of 5.6 μm, and a mean equivalent rounddiameter of 3.2 μm. The water permeability (conditions: differentialpressure 50 kPa, 25° C.) was 0.75 m³/m²·h. The rejection was 70%. Thefracture strength and the fracture elongation were 17.15 MPa and 55%,respectively. Thus, this hollow fiber membrane exhibited inferiorrejection properties.

Example 56

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand, methyl methacrylate homopolymer having aweight-average molecular weight of 280, and γ-butyrolactone were mixedin amounts of 30, 10, and 60 percent by weight, respectively, and thepolymers were dissolved at a temperature of 170° C. This polymersolution was discharged with 100% γ-butyrolactone, acting as a lumenforming fluid, from a double co-extrusion head with a temperature of110° C., and was solidified in a cool bath with a temperature of 28° C.containing an aqueous solution of 80 percent by weight γ-butyrolactone.The product was drawn to 1.5 times in water with a temperature of 80° C.to obtain a hollow fiber membrane. A mixed solution containing 13percent by weight of vinylidene fluoride homopolymer having aweight-average molecular weight of 284 thousand, 5 percent by weight ofpolyethylene glycol having a weight-average molecular weight of 20thousand, 79 percent by weight of dimethylformamide, and 3 percent byweight of water was uniformly applied onto the surfaces of this hollowfiber membrane. The hollow fiber membrane was then immediately immersedin a mixed solvent containing 95 percent by weight of water and 5percent by weight of dimethylformamide. The resulting hollow fibermembrane had a three-dimensional network structure on the external sideand a spherical structure on the internal side. The thickness of thethree-dimensional network structure was 20 μm. This hollow fibermembrane had an outer diameter of 1.59 mm and an inner diameter of 0.95mm. The external surface of the hollow fiber membrane had pores with amean pore size of 0.04 μm. The water permeability (conditions:differential pressure 50 kPa, 25° C.) was 0.55 m³/m²·h. The rejectionwas 99%. The fracture strength and the fracture elongation were 6.76 MPaand 55%, respectively. Thus, the hollow fiber membrane exhibitedexcellent water permeability, rejection properties, strength, andelongation.

Comparative Example 29

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand, methyl methacrylate homopolymer having aweight-average molecular weight of 280, and γ-butyrolactone were mixedin amounts of 30, 10, and 60 percent by weight, respectively, and thepolymers were dissolved at a temperature of 170° C. This polymersolution was discharged with 100% γ-butyrolactone, acting as a lumenforming fluid, from a double co-extrusion head with a temperature of110° C., and was solidified in a cool bath with a temperature of 28° C.containing an aqueous solution of 80 percent by weight γ-butyrolactone.The product was drawn to 1.5 times in water with a temperature of 80° C.to obtain a hollow fiber membrane. The resulting hollow fiber membranehad only a spherical structure. This hollow fiber membrane had an outerdiameter of 1.55 mm and an inner diameter of 0.95 mm. The externalsurface of the hollow fiber membrane had pores with a breadth of 0.96μm, a length of 3.2 μm, and a mean equivalent round diameter of 1.75 μm.The water permeability (conditions: differential pressure 50 kPa, 25°C.) was 0.95 m³/m²·h. The rejection was 79%. The fracture strength andthe fracture elongation were 7.33 MPa and 55%, respectively. Thus, thishollow fiber membrane exhibited inferior rejection properties.

Examples 57 to 60 and Comparative Examples 30 to 32

Hollow fiber membranes were prepared by the following method. First,hollow fiber membranes of Comparative Examples 30 to 32 were preparedunder the conditions shown in Table 7. A vinylidene fluoride homopolymerhaving a weight-average molecular weight of 417 thousand was mixed witha solvent to be dissolved. The solution was discharged with a lumenforming fluid from a double co-extrusion head and solidified in a coolbath. Each hollow fiber membrane has only a spherical structure.

Next, A mixed solution containing 13 percent by weight of vinylidenefluoride homopolymer having a weight-average molecular weight of 284thousand, 5 percent by weight of polyethylene glycol having aweight-average molecular weight of 20 thousand, 79 percent by weight ofdimethylformamide, and 3 percent by weight of water were uniformlyapplied onto the surfaces of the hollow fiber membranes of ComparativeExamples 30 to 32. These hollow fiber membranes were immediatelyimmersed in a mixed solvent containing 95 percent by weight of water and5 percent by weight of dimethylformamide to solidify the solution. Theseare defined as Examples 57 to 59.

A mixed solution containing 9 percent by weight of vinylidene fluoridehomopolymer having a weight-average molecular weight of 284 thousand,3.46 percent by weight of polyethylene glycol having a weight-averagemolecular weight of 20 thousand, 81.44 percent by weight ofdimethylformamide, and 2.1 percent by weight of water was uniformlyapplied onto the surfaces of the hollow fiber membrane of ComparativeExample 32. This hollow fiber membrane was then immediately immersed ina mixed solvent containing 95 percent by weight of water and 5 percentby weight of dimethylformamide. The resulting membrane was defined as

Example 60

These hollow fiber membranes each had a three-dimensional networkstructure on the external side and a spherical structure on the internalside. The thickness of the three-dimensional network structure was 20μm. Table 8 show properties of hollow fiber membranes of Examples 57 to40 and Comparative Examples 30 to 32. Each example exhibited higherrejection properties than those of the comparative examples.

TABLE 7 Ratio of Dissolving Cool bath polymer to temp. Lumen formingHead temp. Cool bath temp. Draw ratio Solvent solvent (° C.) fluidcomposition (° C.) composition (° C.) (Times) Comparative Isophorone40:60 155 100 wt % 100 80 wt % 30 3.0 Example 30 Isophorone Isophoroneaq Comparative Dimethyl sulfoxide 30:70 95  90 wt % 95 90 wt % 8 1.5Example 31 dimethyl sulfoxide aq dimethyl sulfoxide aq Comparativeγ-butyrolactone 38:62 170 100 wt % 95 80 wt % 28 1.5 Example 32γ-butyrolactone γ-butyrolactone aq

TABLE 8 Outer Inner Water Fracture Fracture Base hollow diameterdiameter External surface permeability strength elongation Rejectionfiber membrane (mm) (mm) pore size (μm) (m³/m² · h · 50 kPa) (MPa) (%)property Example 57 Comparative 1.44 0.90 0.17 0.82 9.98 54 96 Example30 Example 58 Comparative 1.99 1.55 0.05 1.19 4.25 32 99 Example 31Example 59 Comparative 1.53 0.85 0.05 0.60 12.79 189 99 Example 32Example 60 Comparative 1.45 0.88 0.03 0.34 12.79 189 99 Example 32Comparative 1.40 0.90 7.1 1.40 10.97 54 30 Example 30 Comparative 1.951.55 2.0 2.05 4.73 32 78 Example 31 Comparative 1.45 0.85 2.3 1.85 15.00189 75 Example 32

Examples 61 to 63

Hollow fiber membranes were prepared in the same manner as in Example 26except that dimethylformamide was replaced with the solvents shown inTable 9. The resulting hollow fiber membranes are defined as Examples 61to 63. Each of these hollow fiber membranes had a three-dimensionalnetwork structure on the external side and a spherical structure on theinternal side. The thickness of the three-dimensional network structurewas 20 μm. Table 9 show properties of hollow fiber membranes of Examples61 to 63. Each example exhibited excellent water permeability, rejectionproperties, strength, and elongation.

TABLE 9 Outer Inner Water Fracture Fracture Rejection diameter diameterExternal surface permeability strength elongation property Solvent (mm)(mm) pore size (μm) (m³/m² · h · 50 kPa) (MPa) (%) (%) Example 61Dimethylacetamide 1.56 0.95 0.05 1.59 12.79 189 98 Example 62N-methyl-2-pyrrolidone 1.56 0.95 0.06 1.91 12.79 189 97 Example 63Dimethyl sulfoxide 1.56 0.95 0.07 2.22 12.79 189 96

Example 64

A vinylidene fluoride homopolymer having a weight-average molecularweight of 417 thousand was mixed with γ-butyrolactone at a ratio of 38to 62 percent by weight, respectively, and was dissolved at atemperature of 150° C. This polymer solution is referred to as A. Inaddition to A, 13 percent by weight of vinylidene fluoride homopolymerhaving a weight-average molecular weight of 284 thousand and 5 percentby weight of polyethylene glycol having a weight-average molecularweight of 20 thousand were mixed with 82 percent by weight ofdimethylformamide, acting as a solvent, and were dissolved at 150° C.This polymer solution is referred to as polymer B. Polymer solutions Aand B were discharged from a double co-extrusion head with a temperatureof 110° C. An aqueous solution of 85 percent by weight γ-butyrolactonewith a temperature of 6° C. and water with a temperature of 20° C. wererespectively sprayed to polymer solutions A and B to solidify thepolymers. The resulting flat membrane had a thickness of 0.175 mm. Oneside of the resulting flat membrane, corresponding to the B side had athree-dimensional network structure and the other side corresponding tothe A side had a spherical structure. Hence, it has been shown that thethree-dimensional network structure and the spherical structure coexist.The mean pore size of the three-dimensional network structure was 1.60μm and the mean diameter of the spherical structure was 4.10 μm. Thesurface of the B side had pores with a mean pore size of 0.06 μm. Thewater permeability at 50 kPa and 25° C. was 0.25 M³/m²·h. The rejectionfor particles having a particle size of 0.843 μm was 98%. The fracturestrength and the fracture elongation were 7.5 MPa and 40%, respectively.Thus, the flat membrane had a dense surface and exhibited excellentwater permeability, rejection properties, strength, and elongation.

Example 65

Using the same polymer solutions A and B as in Example 64, B and A wererespectively discharged from the external pipe and the middle pipe of atriple co-extrusion head while an aqueous solution of 85 percent byweight γ-butyrolactone, acting as a lumen forming fluid, was dischargedfrom the internal pipe. The discharged materials were solidified inwater with a temperature of 6° C. The resulting hollow fiber membranehad an outer diameter of 1.56 mm and an inner diameter of 0.95 mm. Thearea around the external surface had a three-dimensional networkstructure and the area around the internal surface had a sphericalstructure. Hence, it has been shown that the three-dimensional networkstructure and the spherical structure coexist. The mean pore size of thethree-dimensional network structure was 1.60 μm and the mean diameter ofthe spherical structure was 4.10 μm. The external surface had pores witha mean pore size of 0.04 μm. The water permeability at 50 kPa and 25° C.was 1.27 m³/m²·h. The rejection for particles having a particle size of0.843 μm was 99%. The fracture strength and the fracture elongation were12.79 MPa and 189%, respectively. Thus, the hollow fiber membrane had adense surface and exhibited excellent water permeability, rejectionproperties, strength, and elongation.

The porous membrane of the present invention has a high strength andexcellent water permeability and rejection properties. According to themethod for manufacturing the porous membrane of the present invention, aporous membrane can be manufactured through reduced number of steps at alow cost, using a highly chemical-resistant thermoplastic resin. Theresulting porous membrane can exhibit a high strength and excellentwater permeability and rejection properties. The porous membrane of thepresent invention is suitably used for water treatment, batteryseparators, charged membranes, fuel cells, and blood purificationmembranes.

1. A method for manufacturing a porous membrane comprising athree-dimensional network structure and a spherical structure,comprising: simultaneously discharging, from a triple co-extrusion headcomprising an external pipe, a middle pipe and an internal pipe, (A) aresin solution for forming the three-dimensional network structure fromthe external pipe or the middle pipe of the triple co-extrusion head,(B) a resin solution for forming the spherical structure from theexternal pipe of the triple co-extrusion head when the resin solutionfor forming the three-dimensional network structure is discharged fromthe middle pipe or from the middle pipe of the triple co-extrusion headwhen the resin solution for forming the three-dimensional networkstructure is discharged from the external pipe, and (C) a lumen formingfluid from the internal pipe of the triple co-extrusion head; andsolidifying the resin solutions to form the porous membrane.